Jesus appeared to Saint Faustina, a religious in the Congregation of the Sisters of Our Lady of Mercy in Poland, between 1931 to 1938. He expressed to her his desire that the whole world be convinced of His mercy.
The Mission of Saint Faustina consisted in 3 tasks: reminding the world of a truth of our faith revealed in the Holy Scripture about the merciful love of God towards every humans being, even the greatest sinner; in propagating new forms of devotion to the Divine Mercy, such as the veneration of 1) the Image of the merciful Jesus, with the inscription: Jesus, I trust in You, 2) the Feast of the Divine Mercy, 3) the chaplet of the Divine Mercy, mainly at the Hour of Mercy (3pm.), and 4) the spreading the Divine Mercy Devotion ; initiating the apostolic movement of the Divine Mercy which undertakes the task of proclaiming God’s mercy for the world, and strives for childlike confidence in God in fulfilling God’s will and being merciful towards one’s neighbor.
In essence, the devotion to the Divine Mercy consists in an attitude of trust toward God and the charity towards one’s neighbor. The Lord Jesus desires that his creatures trust in Him. “I desire to grant unimaginable graces to those souls who trust in my Mercy (Diary, 687), and that they do works of mercy: through their acts, their words and their prayers.” “You are to show mercy to your neighbors always and everywhere. You must not shrink from this or try to excuse or absolve yourself from it ”(Diary 742).
Jesus appears to Saint Faustina because He desires to save the sinful humanity through His Mercy, thus He said many times to her:
"Mankind will not find solace until it turns with confidence to My Mercy and love. Oh! How much the lack of confidence of a chosen soul hurts Me. This soul recognizes that I am a saint and just, and does not believe in My mercy, does not trust in my love" (Diary, 300).
Jesus told Saint Faustina to "Proclaim that mercy is the greatest attribute of God" (Dairy, 301).
Father Sopocko, the confessor and spiritual director of Saint Faustina, played a special role in her life. He motivated her to write a Diary in order to document the revelations that she had received about the Mercy of God. This diary became an exceptional document of the catholic mystic. Also, thanks to the efforts of the Father Sopocko, The Image of Merciful Jesus was painted, with the inscription "Jesus, I Trust in You". Father Sopocko proclaimed it to the world, as Jesus had requested from Sister Faustina.
Jesus entrusted to Sister Faustina the proclamation of the devotion of His Mercy. "You are the secretary of my mercy; I have chosen you for this charge, in this and in the future life (Diary, 1605), (…) to make known to souls the great mercy that I have for them, and to exhort them to trust in the bottomless depth of my Mercy" (Diary, 1567).
“Today I am sending you with My mercy to the people of the whole world. I do not want to punish aching mankind, but I desire to heal it, pressing it to My Merciful Heart (...) before the Day of Justice I am sending the Day of Mercy”(Diary, 1588)
The main aim of the Divine Mercy is let all mankind know that God is Merciful and desires to heal all souls who trust in Him. The greater the sinner, the greater his mercy; that is why no soul should fear to approach Him.
It is very probable that Pope John Paul II will be called "the Pope of the Mercy" by historians since he offered a strong support for the proclamation of the Divine Mercy.
As Cardinal of Krakow, in 1978 he reversed the prohibition of the devotion to the Divine Mercy, which had been in effect for a period of twenty years (as predicted in Saint Faustina’s diary). The prohibition of this devotion originated as the Holy See could not verify confusing and erroneous translations of St. Faustina’s manuscript, due to the existing political conditions.
Cardinal Karol Wojtyla (later Pope John Paul II), in 1965, began an informative process on the life and virtues of Sister Faustina, which opened the Cause of her Beatification in 1968. Finally, on April 30, 2000 Pope John Paul II, also of Polish origin, solemnly declared Sister Faustina a Saint.
In spite of the initial opposition to the spreading of the Divine Mercy, of Priests, Bishops and laypeople, the devotion to the Divine Mercy has grown and has spread throughout the whole world, as predicted by Jesus in his messages to Sister Faustina. Without doubt, Pope John Paul II was instrumental for this success, and in 1981 he published the Encyclical "Dives in Mercy" (Rich in Mercy), in which he refers to Christ as the “incarnation of the Mercy”, incomprehensible depth of Mercy.